1. Field of the Invention
The present invention relates to a component having a micromechanical microphone structure. The micromechanical microphone structure includes at least one acoustically active diaphragm which functions as a deflectable electrode of a microphone capacitor, a stationary acoustically permeable counterelement which functions as a counter electrode of the microphone capacitor, and means for detecting and evaluating the changes in capacitance of the microphone capacitor.
Moreover, the present invention relates to a method for operating such a microphone component.
2. Description of Related Art
Capacitive microelectromechanical system (MEMS) microphones are becoming increasingly important in various fields of application. This is essentially due to the miniaturized design of such components and the possibility for integrating additional functionalities at very low manufacturing costs. The integration of signal processing components such as filters and components for noise suppression, as well as components for generating a digital microphone signal, is particularly advantageous. Another advantage of MEMS microphones is their high temperature stability.
The diaphragm of the microphone structure is deflected by acoustic pressure. This causes the distance between the diaphragm and the counter electrode to change, resulting in a change in capacitance of the microphone capacitor. These very small changes in capacitance in the AF range must be converted into a usable electrical signal.
One concept frequently implemented in practice is based on charging the microphone capacitor with a direct-current voltage via a high-impedance charging resistor. Changes in capacitance of the microphone capacitor are then detected as fluctuations in the output voltage, which is amplified via an impedance converter. This may be a JFET, for example, which converts the high impedance of the microphone in the range of Gohm into a relatively low output impedance in the range of several 100 ohms without altering the output voltage itself. Instead of a JFET, an operational amplifier circuit may be used which supplies a low output impedance. In contrast to the JFET, in this case the amplifying factor may be adapted to the particular microphone requirements.
The known concept has proven to be problematic in several respects:
The digital circuit elements together with the analog signal processing components are not easily implemented in CMOS technology due to the occurrence of electrical noise. The JFET technology, which requires low noise, cannot be achieved within the scope of standard CMOS processes.
Electrostatic forces of attraction are present between the diaphragm and the counter electrode due to the direct-current voltage applied to the microphone capacitor during operation of the microphone. These electrostatic forces are critical in particular in overload situations, since they promote continuous adherence of the diaphragm to the counter electrode, resulting in a breakdown of the microphone function. To detach the diaphragm from the counter electrode, it is generally necessary to completely discharge the microphone capacitor. In practice, mechanical measures such as a relatively stiff diaphragm suspension, a relatively large distance between the diaphragm and the counter electrode, or mechanical stops, for example, have been attempted in order to avoid such electrostatic collapse. However, these measures usually have an adverse effect on the sensitivity of the microphone, or are very complicated from the point of view of the manufacturing process.
Lastly, it is noted that a relatively high direct-current voltage in the range of 10 volts and greater must be applied to the microphone capacitor to achieve a sufficiently high signal-to-noise ratio (SNR). However, a charging voltage in this range requires a comparatively large distance between the diaphragm and the counter electrode in the range of >>2 μm, on the one hand to avoid electrostatic collapse, and on the other hand to provide a sufficiently large deflection range for the diaphragm. Such large distances are not easily provided using standard surface micromechanical methods. In addition, such high charging voltages in overload situations result not only in adherence of the diaphragm to the counter electrode, but also irreversible melting of the contact surfaces. In practice, attempts have been made to prevent this with the aid of insulating layers. However, these increase the complexity of the manufacturing process, and therefore ultimately increase the costs for such a microphone component.